Li₂CeRbB₁₈O₃₀ and Li₂Cs₂SrB₁₈O₃₀: promising low-birefringence materials for UV and deep UV zero-order waveplates

Zero-order waveplates, which are used to modulate the polarization of light, play a crucial role in scientific instrumentation, optical communications, and the laser industry. However, owing to the limitations imposed by birefringence and absorption edges, only few materials can produce desirable birefringence with suitable transparency in the ultraviolet (UV) and deep ultraviolet (DUV) regions. Borates represent promising materials with widely adjustable birefringence. In this study, we successfully synthesized two new borates, Li₂CeRbB₁₈O₃₀ and Li₂Cs₂SrB₁₈O₃₀, using solid-state reactions in vacuum-sealed tubes; the fundamental building blocks (FBBs) of the two compounds are B₃O₇ and B₉O₁₉ units, respectively. Notably, Li₂CeRbB₁₈O₃₀ and Li₂Cs₂SrB₁₈O₃₀ possess very low birefringence values of 0.0058 at 546 nm and 0.0068 at 546 nm, respectively, considerably lower than that of a commercial quartz waveplate (0.0092 at 532 nm). UV-Vis-NIR diffuse reflectance measurements show that the UV cutoff edges of Li₂CeRbB₁₈O₃₀ and Li₂Cs₂SrB₁₈O₃₀ are lower than 320 and 200 nm, corresponding to the bandgaps of 3.20 and 6.20 eV, respectively. These two compounds can serve as potential zero-order waveplate materials in the UV and DUV regions.